1. Field of the Invention
This invention relates to an engine control system for an internal combustion engine and, more particularly, to an intake and exhaust control system for an internal combustion engine equipped with a mechanical supercharger.
2. Description of Related Art
An internal combustion engine has been developed with improved charging efficiency obtained by means of a supercharger, such as a mechanical supercharger, driven by the engine. The supercharger is typically operated while the engine operates under higher engine loads for high charging efficiency, but is not operated while the engine operates under lower engine loads. Such an engine is known from, for instance, Japanese Unexamined Patent Publication No. 58-220,933. Increasing the charging efficiency increases engine output power and decreases fuel consumption while the engine operates under higher engine loads, but also cause a considerable increase in cylinder temperature and engine knock.
To suppress an increase in cylinder temperature, burned gases may be scavenged in the cylinder with intake air. In order to effectively scavenge burned gases, especially while the engine operates in a high speed range in which engine knock is easily caused, the degree or length of time that an intake valve and an exhaust valve are held open, which is referred as valve overlap, is extended. Such an engine and supercharger combination is known from, for instance, Japanese Unexamined Patent Publication No. 2-119,621.
An internal combustion engine has also been developed with improved charging efficiency obtained by means of a multi-valve mechanism having, for instance, two intake valves for each cylinder. Such a multi-valve internal combustion engine opens either one of the two intake valves when operating in a low speed range and both the two intake valves when operating in a high speed range. This type of an engine and supercharger combination is known from, for instance, Japanese Unexamined Patent Publication No. 61-11,455.
If a multi-valve internal combustion engine is equipped with a mechanical supercharger, engine knocks are easily caused, due to an increase in cylinder temperature. To suppress an increase in cylinder temperature in a multi-valve internal combustion engine, a valve overlap time period between an intake valve and an exhaust valve can be extended so as to cause effective scavenging of burned gases. Further, in order to perform effective scavenging of burned gases by means of intake air forcibly supplied by the supercharger, the valve overlap time period is established so as to be relatively long.
As outlined above, in an internal combustion engine with a mechanical supercharger, in order to perform effective scavenging of burned gases and thereby avoid engine knock, a valve overlap time period may be designed to be relatively long. In such an engine, fuel introduced into a cylinder will be blown out from the cylinder through an exhaust port.
An internal combustion engine equipped with a mechanical supercharger forces a fuel mixture to be blown out from the cylinder through the exhaust ports during the valve overlap period when the engine operates under high load operating conditions in which the mechanical supercharger also operates. Blowing out of the fuel mixture is caused by a supercharged pressure which becomes higher than an exhaust pressure.
Fuel adhering to the wall of the intake port is blown off in an intake stroke, together with the fuel mixture, and mixed in fuel injected by a fuel injector. This results in a slightly richer air fuel mixture than is ideal. Even in an engine having an exhaust system which has a catalytic converter including rhodium (CCRO), during purification of exhaust gases, there is a lack of oxygen in the exhaust system, resulting in an increase in emissions of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gases. Moreover, when the engine operates in a high speed, high load range, since there is an increase in the amount of fuel injected from the fuel injectors, there is a smaller amount of oxygen available in the catalytic converter rhodium than when the engine operates in a low speed, high load range. In order to avoid conditions in which engine knock tends to occur, a valve overlap time period may be designed to be relatively long in a high load region in which the mechanical supercharger is put into operation. Consequently, the engine may produce an increase in the amount of fuel blown out of the cylinder when a relatively high speed is reached, due to operation of the mechanical supercharger. This may result in an increased volume of fuel being discharged from the exhaust system, which increases the amounts of hydrocarbon (HC) and carbon monoxide (CO) in the emissions.